Ethanol production with dilute acid hydrolysis using partially dried lignocellulosics

ABSTRACT

In a process for converting lingnocellulosic biomass to ethanol, the improvement of obtaining higher fermentable soluble sugar yields by drying acid impregnated biomass particles, comprising:  
     a) feeding moist lignocellulosic biomass into an acid impregnator to render it acid-soaked and draining the acid-soaked biomass to about 30% to 35% by weight solids;  
     b) dewatering the acid-soaked biomass by drying or centrifugation to prevent compaction of the biomass and arrive at about 40% to 60% by weight solids;  
     c) subjecting the acid-impregnated biomass to a first-stage hydrolysis reactor at a temperature of from 130° C. to 220° C. and discharging formed hydrolysate into a flash tank at about 120° C. to 140° C. to hydrolyze most of the remaining soluble oligosaccharides to monomeric sugars, and flashing remaining hydrolysate to a second flash tank at a lower temperature than the first flash tank—the second flash tank serving as a feed surge tank for a counter-current extractor;  
     d) washing the hydrolysate, adjusting the pH of the sugar extract to about 5, and recovering more than 95% of the soluble sugars in the first-stage hydrolysate slurry by a counter-current extractor;  
     e) subjecting remaining washed-first stage solids of pretreated biomass to a second-stage acid and metal salt impregnator and dewatering by drying or centrifugation to prevent compaction of biomass to arrive at 40% to 60% by weight solids;  
     f) subjecting the acid and metal salt-impregnated biomass to a second-stage hydrolysis reactor at a temperature from 190° C. to 240° C. and discharging formed hydrolysate into a flash tank, at about 120° C. to 140° C. to hydrolyze most of the remaining soluble oligosaccharides to monomeric sugars and flashing remaining hydrolysate to a second flash tank at a lower temperature than the first flash tank, the second flash tank serving as a feed surge tank for second-stage fementors;  
     g) cooling pH-adjusted extract from the counter-current extractor, feeding the extract to a first-stage fermentor and air sparging the first-stage fermentor at a rate sufficient to promote enough yeast growth to compensate for loss through second-stage fermentors;  
     h) pH adjusting second-stage hydrolysate slurry to 4.5, cooling the slurry and adding it into the top of the first fermentor of a two-fermentor train in the second stage fermentors, pumping broth from the bottom of the first stage fermentors to the second stage fermentors while the yeast is in the growth phase for a period sufficient to consume over 95% of fermentable sugars; and  
     i) recovering ethanol.

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government has rights to this inventionpursuant to Contract No. DE-AC36-99GO-10337 between the United StatesDepartment of Energy and the Midwest Research Institute.

TECHNICAL FIELD

[0002] The invention is a continuation-in-part of U.S. Pat. applicationSer. No. 09/634,978 filed Aug. 9, 2000, and relates to hydrolyzinglignocellulosic materials by subjecting dried lignocellulosic materialin a reactor to a catalyst comprised of a dilute solution of a strongacid and a metal salt to lower the activation energy (i.e., temperature)of cellulose hydrolysis and ultimately obtain higher sugar yields. Thelower temperature obtained occasions a reduction in the cost of steamand equipment and enables the hydrolysis of both hemicellulose andcellulose, when used with hydrolyzer feeders that do not compact thebiomass feedstock to achieve higher sugar yields.

[0003] Lignocellulose is ubiquitous in all wood species and allagricultural and forestry waste. In addition, municipal waste, whichtypically contains about half waste paper and yard waste, is a source oflignocellulosic materials. Currently, municipal waste is buried orburned at considerable expense to the disposer or the governmentorganization providing solid waste services.

[0004] Lignocellulosic biomass is a complex structure of cellulosefibers wrapped in a lignin and hemicellulose sheath. The ratio of thethree components varies depending on the type of biomass. Typical ratiosare as follows: Softwoods Corn Cobs RDF* Cellulose 42% 40% 52%Hemicellulose 25% 36% 26% Lignin 28% 13% 20%

[0005] Different woods also have different compositions. Softwoods(gymnosperms) generally have more glucomannan and less glucuronoxylanthan hardwoods and grasses (angiosperms).

[0006] Cellulose is a polymer of D-glucose with β [1Õ4] linkages betweeneach of the about 500 to 10,000 glucose units. Hemicellulose is apolymer of sugars, primarily D-xylose with other pentoses and somehexoses with β [1Õ4] linkages. Lignin is a complex random polyphenolicpolymer. Therefore, lignocellulose represents a very cheap and readilyavailable substrate for the preparation of sugars, which may be usedalone or microbially fermented to produce alcohols and other industrialchemicals.

[0007] Ethanol, one of the alcohols, which can be produced fromlignocellulosic biomass, has a number of industrial and fuel uses. Ofparticular interest is the use of ethanol as an additive to gasoline toboost octane, reduce pollution and to partially replace gasoline in themixture. This composition is the well-known commercial product called“gasohol.” It has been proposed to eliminate gasoline completely fromthe fuel and to burn ethanol alone. Such a fuel would produceconsiderably less air pollution by not forming as much carbon monoxideor hydrocarbon emissions. Furthermore, gasoline is produced from crudeoil; which fluctuates in price, availability, and is the subject ofunpredictable world politics.

[0008] It has been estimated that about 1×10⁹ tons of lignocellulosicwastes are produced every year. This amount exceeds the total amount ofcrude oil consumed per year. In theory, if properly managed, thelignocellulose produced by the United States is sufficient to produceall of the country's needs for liquid fuel if the cellulose andhemicellulose can be completely converted into ethanol. The amount ofenergy theoretically obtainable from the combustion of cellulose or theglucose or alcohol derived therefrom is about 7200 BTU per pound orroughly equivalent to 0.35 pounds of gasoline. Hemicellulose has similarvalue when converted into sugars or ethanol. Consequently, cellulose andhemicellulose represent a readily available potential source for ethanolproduction. The technology for the production of ethanol from grain andfruit for beverage purposes has been well developed for centuries.However, the costs have been relatively high compared to the cost ofgasoline. Accordingly, many methods have been proposed to reduce thecost and increase the efficiency of ethanol production.

[0009] Among the techniques proposed for the production of fuel gradeethanol include the hydrolysis of cellulose and hemicellulose to producesugars which can be fermented to produce ethanol. Cellulose in the formof wood, newsprint and other paper, forest, agricultural, industrial andmunicipal wastes is quite inexpensive compared to grain, fruit, potatoesor sugarcane which is traditionally used to prepare alcohol beverages.

[0010] Hydrolysis of lignocellulosic biomass using an acid catalyst toproduce sugars has been known for decades but can be costly and requiresspecial equipment. The hydrolyzed sugars themselves are somewhat labileto the harsh hydrolysis conditions and may be degraded to unwanted ortoxic byproducts. If exposed to acid for too long, the glucose derivedfrom cellulose degrades into hydroxymethylfurfural, which can be furtherdegraded into levulinic acid and formic acid. Xylose, a hemicellulosesugar, can be degraded into furfural and further to tars and otherdegradation products.

[0011] In order for acid to completely hydrolyze the cellulose andhemicellulose in a lignocellulosic substrate, degradation of thedesirable sugars and formation of the toxic byproducts cannot be avoideddue to kinetic constraints. On the other hand, to use conditionssufficiently gentle that significant degradation of sugars will notoccur does not result in complete hydrolysis of substrate. Furthermore,the acid is corrosive and requires special handling and equipment.Accordingly, in the last twenty years attention has focused on enzymatichydrolysis of cellulose with cellulase followed by fermentation of theresulting sugars to produce ethanol which in turn is distilled to purifyit sufficiently for fuel uses.

[0012] Cellulase is an enzyme complex that includes three differenttypes of enzymes involved in the saccharification of cellulose. Thecellulase enzyme complex produced by Trichoderma reesei QM 9414 containsthe enzymes named endoglucanase (E.C. 3.2.1.4), cellobiohydrolase(E.C.3.2.1.91) and β-glucosidase (E.C.3.2.1.21). Gum et al. Biochem.Biophys.Acta, 446:370-86 (1976). The combined synergistic actions ofthese three enzymes in the cellulase preparation completely hydrolysescellulose to D-glucose.

[0013] However, cellulase cannot completely degrade the cellulose foundin native, unpretreated lignocellulose. It appears that thehemicellulose and lignin interfere with the access of the enzyme complexto the cellulose, probably due to their coating of the cellulose fibers.Furthermore, lignin itself can bind cellulase thereby rendering itinactive or less effective for digesting cellulose. For example, rawground hardwood is only about 10 to 20% digestible into sugars using acellulase preparation.

BACKGROUND ART

[0014] U.S. Pat. No. 4,529,699 discloses a process for obtaining ethanolby continuous acid hydrolysis of cellulosic materials by providing ahomogenized slurry of heated (160° to 250° C.) cellulosic materialcontinuously into a reactor, adding concentrated acid to the pressurizedand heated cellulosic material to obtain hydrolysis, neutralizing andfermenting the resulting aqueous solution to obtain ethanol, andrecovering resulting byproducts of methanol, furfural, acetic acid andlignin.

[0015] A process for the production of sugars and optionally celluloseand lignin from lignocellulosic raw materials is disclosed in U.S. Pat.No. 4,520,105. The process entails subjecting vegetable materials to achemical pretreatment with a mixture of water and lower aliphaticalcohols and/or ketones at 100° C. to 190° C. for a period of from 4hours to 2 minutes with control of the breakdown of the hemicellulosecomponents followed by separation of residue and a subsequent chemicaltreatment with a similar solvent mixture at elevated temperatures for aperiod of from 6 hours to 2 minutes.

[0016] U.S. Pat. No. 5,411,594 discloses a hydrolysis process system forcontinuous hydrolysis saccharification of lignocellulosics in atwo-stage plug-flow-reactor system. The process utilizes dilute-acidhydrolysis and is primarily by reverse inter-stage transfer-flow,opposite to biomass, of second-stage surplus of: process heat;dilute-acid; and ingredient and solution water, all in an alphacellulose hydrolysate, dilute-acid solution. The primary final productis the combined hydrolysate sugars in a single solution, includingpentose, hexose and glucose sugars, which are fermented into ethanoland/or Torula yeast. The secondary final solid product is anunhydrolyzed lignin solid.

[0017] A method of treating biomass material using a two-stagehydrolysis of lignocellulosic material is disclosed in U.S. Pat. No.5,536,325. The conditions during the first stage is such as to hydrolyzeor depolymerize the hemicellulosic component without substantialdegradation of resulting monosaccharides and conditions during thesecond stage being such as to hydrolyze the cellulose to glucose withoutsubstantial degradation of the glucose. Hydrolysis in both stages isaccomplished by the use of nitric acid, and the pH, retention time, andtemperature in both stages are selected to maximize production of thedesired monosaccharide or monosaccharides.

[0018] U.S. Pat. No. 6,022,419 discloses a multi-function process forhydrolysis and fractionation of lignocellulosic biomass to separatehemicellulosic sugars from other biomass components such as extractivesand proteins; a portion of the solubilized lignin; cellulose; glucosederived from cellulose; and insoluble lignin form the biomass byintroducing a dilute acid into a continual shrinking bed reactorcontaining a lignocellulosic material at 94° to 160° C. for 10 to 120minutes at a volumetric flow rate of 1 to 5 reactor volumes tosolubilize extractives, lignin, and protein by keeping thesolid-to-liquid ratio constant throughout the solubilization process.

[0019] A process for rapid acid hydrolysis of lignocellulosic materialis disclosed in U.S. Pat. No. 5,879,463. The process is a continuousprocess for acid hydrolysis of lignocellulosic material through whichdelignification and saccharification are carried out in a singlereaction cycle employing a solubilizing organic solvent of lignin and astrong and extremely diluted inorganic acid to obtain highlyconcentrated recoveries of sugar.

[0020] There is a need in the art of using lignocellulosic materials toobtain fermentable sugars for production of ethanol, to develop moreeffective pretreatment methods that result in high hemicellulose sugaryield and high enzymatic cellulose digestibility, all of which result ingreater yields of ethanol.

DISCLOSURE OF INVENTION

[0021] One object of the present invention is to provide more effectivelignocellulosic pretreatment methods that entail drying acid-impregnatedlignocellulosic biomass, that result in higher hemicellulose sugaryields and higher enzymatic cellulose digestibility en route toproducing ethanol.

[0022] Another object of the present invention is to providepre-hydrolysis conditions for lignocellulosic materials by subjecting anacid-soaked feedstock to drying (using heated gas such as air, nitrogenor carbon dioxide, or superheated steam, or any combination of heatedgas and steam), which is believed to reduce compaction and therebylessen collapse of cells in pressed chips to permit good mass and heattransfer to achieve even cooking and higher overall sugar for productionof ethanol.

[0023] A further object of the present invention is to provide posthydrolysis fermentation conditions for dried lignocellulosic materialsusing a two-stage fermentation process, wherein the first fermentationoperates under microaerobic conditions to maintain adequate yeast cellconcentration that is forwarded to a second-stage fermentor during thegrowth phase to enable the yeast to achieve 90% ethanol yield fromfermentable sugars without the need for detoxification of thehydrolysate liquor.

[0024] In general, the invention process for converting lignocellulosicbiomass to ethanol employs: a two-stage dilute acid hydrolysis processthat hydrolyzes partially dried, acid-impregnated lignocellulosicbiomass to fermentable sugars; a countercurrent extraction process torecover over 95% of soluble sugars from the first-stage hydrolysate withminimal dilution of sugar solution; and a two-stage fermentation processwhich incorporates yeast recycle in the first-stage liquid fermentors(the first fermentor operates under microaerobic conditions to maintainadequate yeast cell concentration, wherein a first-stage fermentationbroth is forwarded to second-stage fermentors during the growth phase ofthe yeast, and the second-stage fermentation is carried out in slurryfermentors).

[0025] The process enables the yeast to achieve 90% ethanol yield fromfermentable sugars without the need for detoxification of thehydrolysate liquor. This adaptation method also reduces nutrientrequirements. Furthermore, the second-stage slurry fermentationeliminates the need for washing the second-stage hydrolysate to recoversoluble sugars.

BRIEF DESCRIPTION OF DRAWINGS

[0026]FIG. 1 is a process block flow diagram showing production ofethanol from the invention process using partially dried lignocellulosicbiomass.

[0027]FIG. 2 is a graph depicting xylose yield from first-stage diluteacid hydrolysis of corn stover when hydrolyzed with dilute sulfuric acidat 180° C. to 190° C.

[0028]FIG. 3 is a graph depicting hemicellulose sugar yield fromfirst-stage and glucose yield from second-stage from a dilute solutionof a strong acid hydrolysis of whole-tree softwood forest thinnings.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] Some major hurdles in the development of commercially feasiblebiomass-to-ethanol processes are: (i) the high cost of cellulaseenzymes; and (ii) the need to develop effective pretreatment methodsthat result in high hemicellulose sugar yield and high enzymaticcellulose digestibility.

[0030] Currently, two-stage dilute acid hydrolysis appears to be theleading technology; however, while this technology has been demonstratedat pilot and commercial scale, improvements in sugar and ethanolconversion yields are necessary to make the process commercially viableeven where low-cost feedstock is available. In this connection, mosttwo-stage dilute acid hydrolysis processes developed to date require adetoxification step (usually by overliming) to improve thefermentability of the hydrolysates and ethanol yield. The overlimingmethod produces a large quantity of gypsum, which poses a disposalproblem.

[0031] We have developed a method for adapting yeast to the inhibitorsin softwood hydrolysates so that high ethanol yield (90%) can beobtained without overliming requirements. The method employs two mutantSaccharomyces cerevisiae yeasts.

[0032] We have also adapted a xylose fermenting yeast strain Pichiastipitis to softwood hydrolysates. The yeast adaptation method isincorporated in the overall process design by including a yeast recycleloop in the first-stage fermentation.

[0033] Another challenge in the development of commercially feasiblebiomass-to-ethanol processes is scaling up the hydrolysis reactors. Atpresent, most commercial scale reactors are batch percolators. Althoughseveral continuous pilot hydrolyzers are in operation (e.g., TennesseeValley Authority, Muscle Shoals, Ala., and the National Renewable EnergyLaboratory, Golden, Colo.), these continuous reactors are yet to beproven, especially at high pressure and high temperature conditions usedin the second stage.

[0034] Research on two-stage dilute acid hydrolysis processes forconverting softwood forest thinnings has been performed in a 4-L batchsteamer digester (also called a steam gun). Hemicellulose sugar yieldsof 85%-90% and glucose yields of 55%-60% were achieved. Processsimulation and cost evaluation indicate that the amount of water used inthe process (such as water added in the acid impregnation step and washwater used to recover hemicellulose sugars from first-stage hydrolysate)has a significant impact on the production cost. Although some processwater can be recycled, large volumes of water in the system would resultin larger equipment size and lower product concentrations. Therefore, itis critically important to minimize the amount of water added to thehydrolyzers (i.e., water entrained in biomass feed) and thehemicellulose sugar extraction system (i.e., wash water).

[0035] Because of the low cost and relative ease of handling, sulfuricacid is used in the two-stage dilute acid hydrolysis process. Acidimpregnation is achieved by soaking the biomass in dilute acid solution,under elevated temperature and pressure. Excess acid solution is thenremoved from the biomass, normally by pressing via a screw press. Mostcontinuous biomass hydrolyzers (such as the Sunds Hydrolyzer, the PandiaReactor, and the Stake II Reactor) employ screw feeders to feed biomassinto the reactor under pressure. These compression feeders create adense biomass plug to seal the reactor. The pressure in the screwfeeders (i.e., certain Sunds screw feeders) can reach as high as 1,200psig. At this pressure the solid content of sawdust particles isincreased to about 70%.

[0036] Experimental results clearly show that pressing acid-soakedbiomass from a starting solid content of 33% to 45% before hydrolysisusing the batch digester lowers the soluble hemicellulosic sugar yieldapproximately 10% in comparison with biomass that are air-dried to thesame solids content. This is clearly shown in FIG. 2 where air drying incomparison with pressing acid-soaked corn stover to the same solid andacid contents generally produces higher xylose yields in dilute acidhydrolysis. The Combined Severity Factor (Log Ro-pH)¹ is used todescribe the severity of dilute acid catalyzed steam pretreatment. Thereaction ordinate Ro² is given by the following expression:

Ro=t*exp[(T−100)/14.75]

[0037] where t is the reaction time in minutes;

[0038] T is the reaction temperature in degree Celsius;

[0039] * is multiplication; and

[0040] exp is the exponential

[0041] The acid concentrations in both types of chips are essentiallythe same in this comparison test.

[0042] The difference in sugar yield is even more dramatic for woodchips as shown in FIG. 3, which depicts hemicellulose sugar yield fromfirst stage and glucose yield from second stage dilute acid hydrolysisof whole-tree softwood forest thinnings. One possible explanation forthe lower sugar yield for pressed chips is that some cells in thepressed chip collapse. As a result, good mass and heat transfer are notachieved in compacted chips, which leads to uneven cooking of the batchand lower overall sugar yield. The implication of this observation isimportant regarding the selection of hydrolyzer and feeder design.

[0043] Continuous compression feeders, which compact biomass particlesand collapse cell walls tend to lead to uneven heating of biomassparticles and lower sugar yield. To ensure the integrity of theacid-impregnated biomass particles, certain features have been includedin the process design. These include an acid impregnator which does notsqueeze the biomass to higher than about 35% solids content, anair-drier (or a centrifuge) to increase the solids content ofacid-soaked biomass to about 45% solids, and a hydrolyzer design whichdoes not use compression screw feeders. Batch or continuous steamdigesters using non-pressurized gravity feeders or pressurized gravityfeeders (such as pressurized lock-hoppers) or rotary valve feeders aremost suitable for application in both first-and second-stage hydrolysis.

[0044] A countercurrent extractor is used to recover soluble sugars fromfirst-stage hydrolysate with minimal use of wash water. To preserve theintegrity of insoluble solids, the extractor is controlled so as not tosqueeze the solid particles to higher than about 35% solids. Acountercurrent screw-conveyor extractor or a vacuum belt washer issuitable for recovering soluble sugars from first-stage hydrolysate. Thescrew extractor is more efficient in water usage, and in recognitionthat it is very difficult and costly to wash soluble sugars from themud-like second-stage hydrolysate, no second-stage washers have beenemployed. Instead, the second-stage hydrolysate is slurried with thefirst-stage fermentation broth and sent to the second stage fermentors.

EXAMPLES

[0045] First-Stage Acid Impregnation

[0046] Moist biomass feedstock of between 35% and 50% by weight solidscontent is fed into an acid impregnator. The feedstock may consist ofwood chips, sawdust, milled agricultural residues, or corn-refiningresidue. The design of the acid impregnator is dependent upon the typeof acid used. If gaseous sulfur dioxide is used, no water is added. Ifanother strong acid such as sulfuric, hydrochloric, or nitric or anystrong acid which effect pH values below about 3, is used, a dilutesolution of one of these acids is heated from about 40° C. to about 80°C. before adding to the impregnator. Optionally, a small amount of ametal salt catalyst (such as ferrous sulfate) is added in an amountsufficient to provide higher overall fermentable sugar yields than isobtainable when hydrolyzing with dilute acid alone. In order tothoroughly soak the biomass in the acid solution, a residence time ofabout 1 to 3 hours is required. For gaseous sulfur dioxide, the contacttime is shorter or in the vicinity of less than 30 minutes. Theacid-soaked feedstock is drained or squeezed to about 35% solids uponexiting the impregnator. The feedstock is further dewatered to about 40%to about 60% solids using either a dryer or a centrifuge (if gaseous SO₂is used and if the solid content of biomass feedstock is between about40% and about 60% by weight, dewatering is not necessary). A dryer(using heated air or gas such as nitrogen or carbon dioxide, superheatedsteam or any combination of steam and heated air or gas) is superiorbecause the centrifuge tends to cause compaction of biomass particles.If sulfuric acid is used, the acid concentration of the liquid in thebiomass prior to feeding into the first-stage hydrolyzers is in therange of from about 0.3% to about 4.0% by weight.

[0047] First-Stage Hydrolysis

[0048] The acid-impregnated biomass is fed into the first-stagehydrolyzer via a non-pressurized or pressurized gravity feeder or rotaryvalve feeder that does not compact or densify the biomass particles. Thehydrolyzer may be a batch or continuous hydrolyzer. Steam is directlyinjected into the hydrolyzer in order to heat the biomass to the desiredtemperature. The hydrolysis is conducted at a temperature of from about130° C. to about 220° C. for a period of from about 1 to about 60minutes. Thereafter, the hydrolysate is discharged into a flash tankoperating at a temperature of from about 120° C. to about 140° C., andis held there for a period of from about 1 to about 3 hours to hydrolyzemost of the soluble oligosaccharides to monomeric sugars. Thehydrolysate from the first flash tank is then flashed into a secondflash tank operating at a temperature of about 95° C. The second flashtank serves as a feed surge tank for the countercurrent extractor.

[0049] Counter-Current Extractor

[0050] In excess of 95% of soluble sugars from the first-stagehydrolysate slurry is recovered by the counter-current extractor (whichmay be a screw-conveyor extractor or a vacuum belt extractor). Thehydrolysate is washed with hot water at a temperature of from about 50°C. to about 80° C., wherein the water is used in a ratio of from about4/1 for liquid-to-insoluble solids. Alkali (lime or ammonia) is added tothe extract to bring the pH to about 5. If lime is used, theprecipitates (mostly gypsum) are filtered out of the extract and thefiltrate is forwarded to the first stage fermentors. The extractedsolids are dewatered to about 35% solids, and conveyed to thesecond-stage acid impregnator.

[0051] Second-Stage Acid Impregnation

[0052] The second-stage acid impregnator may be similar in design to thefirst-stage impregnator. The insoluble solids from the extractor aresoaked in an aqueous solution of a dilute acid catalyst and a metal saltcatalyst sufficient to provide higher overall fermentable sugar yieldsthan is obtainable when hydrolyzing with dilute acid alone. A strongacid such as sulfuric, hydrochloric, nitric, SO₂ or any strong acid,which effect pH values below about 3 and a metal salt catalyst selectedfrom the group consisting of ferrous sulfate, ferric sulfate, ferricchloride, aluminum sulfate, aluminum chloride, and magnesium sulfate,may be used.

[0053] The acid-soaked biomass (washed first-stage solids) is dewateredusing a dryer or a centrifuge to about 45% solids. If gaseous SO₂ isused, the washed solids are impregnated with an aqueous solution of ametal salt catalyst, then dried to about 45% solids before entering theSO₂ impregnator. The dryer is preferred due to the risk of compaction ofbiomass particles when using the centrifuge. The acid concentration ofliquid in the acid-impregnated biomass prior to feeding into thesecond-stage hydrolyzers is in the range of from about 0.5 to 4% byweight, and the concentration of the metal salt catalyst is betweenabout 0.2 mmole/L and about 25 mmole/L.

[0054] Second-Stage Hydrolysis

[0055] Acid and metal salt-impregnated biomass is fed into thesecond-stage hydrolyzer using gravity feeders or rotary valve feedersthat do not densify the biomass particles. The hydrolyzer may be batchor continuous. To heat the biomass to the desired temperature, steam isinjected directly into the hydrolyzer to create hydrolysis temperaturesof from about 190° C. to about 240° C. for a period of from about 1 toabout 10 minutes. The hydrolysate is then discharged into a flash tankoperating at a temperature of from about 120° C. to about 140° C. for aperiod of from about 1 to about 3 hours to hydrolyze most of the solubleoligosaccharides to monomeric sugars. The hydrolysate from the firsttank is then flashed into a second flash tank operating at a temperatureof about 95° C. The second flash tank serves as a feed surge tank forthe second-stage ferementors.

[0056] Ethanol Fermentation

[0057] Ethanol fermentation is carried out in two stages to incorporatethe yeast adaptation and recycle feature.

[0058] First-Stage Fermentation:

[0059] The pH-adjusted and filtered extract from the counter-currentextractor is cooled to about 32° C. to about 42° C. depending upon yeaststrain and adaptation, and fed to the bottom of the first fermentor of atwo-fermentor train connected in series. Each fermentor has a residencetime of about 8 hours. Air is sparged into the bottom of the firstfermentor (through air distributors) at a rate equivalent to about 0.05volume of air per fermentor volume per minute (vvm). Air additionpromotes some yeast growth to make up for the loss through thesecond-stage fermentors. The first fermentor is equipped with aside-entry mixer to keep the yeast cells in suspension, although sideentry is not a necessary requirement of this invention. Corn steepliquor and ammonium sulfate may be added as nutrients to the feed streamto promote yeast growth. The broth from the first fermentor overflowsinto the top of the second fermentor. Inclined plates and a yeast sumpare installed in the bottom of the second fermentor to facilitate yeastseparation. Most of the yeast settling in the yeast sump is pumped backin with the feed into the first fermentors. Broth from the secondfermentor (still containing some yeast) is forwarded to the first of thesecond-stage fermentors. The residence time of the first-stagefermentation is varied by controlling the liquid level in the secondfermentor. The residence time is controlled such that yeast leaving thesecond fermentor in the first-stage fermentation train is in the growthphase. When softwood hydrolysate is used, the first-stage fermentationresidence time is approximately 16 hours. Yeast is the preferredfermenting organism. The first fermentor may be seeded with one or amixed culture of hexose-fermenting yeast and xylose-fermenting yeast.

[0060] Second-Stage Fermentation:

[0061] Alkali such as lime or ammonia is added to the second-stagehydrolysate slurry to adjust the pH to about 4.5. The slurry is cooled,using a slurry cooler, to about 32° C. to 42° C. depending upon yeaststrain and adaptation method, and then mixed with the first-stagefermentation broth. Thereafter, the slurry is fed into the top of thefirst fermentor of a two-fermentor train. Broth exiting the firstfermentor in the second-stage fermentation train at the bottom is pumpedto the top of the second fermentor. Both fermentors are equipped withside-entry mixers to keep the insoluble solids in suspension. Side entrymixers are not essential to this invention. The residence time in eachfermentor is about 8 hours. At the end of the second-stage fermentation,more than 95% of the fermentable sugars have been consumed. Thefermentation broth is then pumped into a beer well, which serves as asurge tank for both fermentation and distillation systems.

[0062] Distillation

[0063] Ethanol is recovered from the beer by conventional distillationmethods. The trays of the beer column are designed to handle theinsoluble solids.

[0064] Insoluble Solids Recovery

[0065] The beer column bottom stream is centrifuged to recover most ofthe suspended solids. The centrifuge cake is further dewatered toapproximately 50% total solids using a press (filter press, belt pressor screw press) before being sent to the biomass boiler.

[0066]FIG. 1 is a block flow diagram of the production of ethanol fromlignocellulosic biomass using the two-stage dilute acid process, asmodified by the present invention.

[0067] In the process of FIG. 1, the drying step as shown by the blockwith the single asterisk is not necessary if gaseous SO₂ is used in thefirst-stage acid impregnation step. However, if the gaseous acid SO₂ isused, the washed pretreated biomass material is impregnated with asolution of a metal salt catalyst, then dried as shown by the block withthe double asterisks to about 45% solids before entering thesecond-stage acid impregnator (i.e., the drying step precedes theacid-impregnation step).

[0068] The single asterisk in FIG. 1 is to signify that drying is notnecessary if gaseous SO₂ is used in the first-stage acid impregnation.

[0069] The double asterisk in FIG. 1 indicates that, if gaseous SO₂ isused, the washed biomass material is dried to about 40%-60% solidsbefore entering the acid impregnator (i.e., the drying steps precedesthe acid-impregnation step).

1. In a process for converting lignocellulosic biomass to ethanol, theimprovement of obtaining higher fermentable soluble sugar yields bydrying acid impregnated biomass particles, comprising: a) feeding amoist lignocellulosic biomass feedstock into a dilute strong acidimpregnator to effect ph values below about 3 for a sufficient residencetime to render it acid-soaked and draining the acid-soaked biomass toabout 30% to about 35% by weight solids; b) dewatering said acid-soakedbiomass by drying or centrifugation in a manner so as to preventdensifying the biomass particles and obtain a solids content of about40% to 60% wet weight basis; c) subjecting said acid-impregnated biomassto a first-stage hydrolysis reactor at a temperature sufficient tocommence hydrolysis and discharging the formed hydrolysate into a firstflash tank at a temperature sufficient to hydrolyze most of the solubleoligosaccharides to monomeric sugars and flashing remaining hydrolysateto a second flash tank at a lower temperature than the first flashtank—said second flash tank serving as a feed surge tank for acounter-current extractor; d) washing the hydrolysate, recovering morethan about 95% of the soluble sugars in the first-stage hydrolysateslurry by a counter-current extractor, and adjusting the pH of theextract to about 5; e) subjecting remaining washed first stagepretreated solids to a second-stage acid and metal salt impregnator anddewatering by drying or centrifugation in a manner so as to preventcompaction of the biomass particles and obtain a solids content of about40% to 60% wet weight basis; f) subjecting said acid and metalsalt-impregnated biomass to a second-stage hydrolysis reactor at atemperature of from about 190° C. to about 240° C. and discharging theformed hydrolysate into a flash tank at about 120° C. to about 140° C.to hydrolyze most of the remaining soluble oligosaccharides to monomericsugars and flashing remaining hydrolysate to a second flash tank at alower temperature than the first flash tank—said second flash tankserving as a feed surge tank for second-stage fermentors; g) cooling thepH-adjusted extract from said counter-current extractor, feeding theextract to a first-stage fermentor and air sparging the first-stagefermentor at a rate sufficient to promote enough yeast growth tocompensate for loss through second-stage fermentors; h) pH adjustingsecond-stage hydrolysate slurry to about 4.5, cooling the slurry andadding it into the top of the first fermentor of the second-stagefermentation train, pumping broth from the bottom of the first fermentorto the top of a second fermentor for a period sufficient for the carriedover yeast to consume over about 95% of fermentable sugars; and i)recovering ethanol.
 2. The process of claim 1 wherein said reactor is abatch steam explosion reactor.
 3. The process of claim 1 wherein saidreactor is a continuous reactor.
 4. The process of claim 1 wherein saidlignocellulosic feedstock is selected from the group consisting ofsoftwood, hardwood, agricultural residues, corn refining residues andgrasses.
 5. The process of claim 4 wherein said softwood is Douglas Fir,White Fir, Ponderosa Pine, Sitka spruce, and Hemlock.
 6. The process ofclaim 1 wherein said dilute acid is selected from the group consistingof H₂SO₄, HCl, HNO₃, SO₂, and said metal salt catalyst is selected fromthe group consisting of ferrous sulfate, ferric sulfate, ferricchloride, aluminum sulfate, aluminum chloride, and magnesium sulfate. 7.The process of claim 6 wherein said acid is H₂SO₄ and said residencetime is about 1 to about 60 minutes for first stage hydrolysis and about1 to about 10 minutes for second stage hydrolysis.
 8. The process ofclaim 7 wherein said temperature sufficient to commence hydrolysis isfrom about 130° C. to about 220° C. for first stage hydrolysis, and fromabout 190° C. to about 240° C. for second stage hydrolysis.
 9. Theprocess of claim 8 wherein said temperature sufficient to hydrolyze mostof the soluble oligosaccharides to monomeric sugars in first flash tankis from about 120° C. to about 140° C., and the residence time ofhydrolysate in the flash tank is from about 1 to about 3 hours.
 10. Theprocess of claim 6 wherein said acid is SO₂ and said residence time isabout 1 to about 60 minutes.
 11. The process of claim 10 wherein saidtemperature sufficient to commence hydrolysis is from about 130° C. toabout 220° C. for first stage hydrolysis, and from about 190° C. toabout 240° C. for second stage hydrolysis.
 12. The process of claim IIwherein said temperature sufficient to hydrolyze most of the solubleoligosaccharides to monomeric sugars in first flash tank is from about120° C. to about 140° C., and the residence time of hydrolysate in theflash tank is from about 1 to about 3 hours.
 13. The process of claim 6wherein said acid is H₂SO₄, dewatering is by drying, and the acidconcentration in the liquid in, the biomass before subjecting to firststage or second stage hydrolysis is between about 0.3% to about 4.0% byweight, and the concentration of said metal salt is between about 0.2 toabout 25.0 mmole/L.
 14. The process of claim 6 wherein said acid is SO₂and the feedstock is dewatered (if necessary) to 40% to 60% wet weightbasis prior to SO₂ impregnation, and the acid concentration in theliquid in the biomass before subjecting to first stage or second stagehydrolysis is between about 0.3% to about 6.0% by weight.
 15. Theprocess of claim 1 wherein in step g) air sparging in the firstfermentor of the first-stage fermentation train sufficient to promoteyeast growth is about 0.05 vvm.
 16. The process of claim 1 wherein instep g) air sparging sufficient to promote yeast growth is about 0.05vvm.
 17. The process of claim 1 wherein in step f) said lowertemperature in said second flash tank is about 95° C.
 18. The process inclaim 1 wherein in step h) said whole slurry from the second stagehydrolysis reactor is fermented in the second train of fermentors usingyeasts from the first stage fermentors.
 19. The process of claim 1wherein in step a) gaseous SO₂ is used as the acid, the solid content ofbiomass is between about 40% to about 60% by weight, and the dewateringstep b) is omitted.
 20. The process of claim 1 wherein said drying isdone using heated air, nitrogen, carbon dioxide, superheated steam orany combination thereof.
 21. The process of claim 1 wherein in step d)said extract is recovered hydrolysate liquor.
 22. The process of claim 1wherein in step e) said pretreated solids are insoluble solids.
 23. Theprocess of claim 4 wherein said municipal solid waste is constructionlumber.